Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already mark it read-only, idempotent, and open-world, but the description adds substantial context beyond that: pricing ($0.01, USDC on Base), the x402 payment handshake, latency (~1-3 s), and the shape of the result (grade, spoofable flag, fixes). This is exactly the behavioral detail an agent needs before invoking.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Dense but front-loaded: purpose is stated first, then payment mechanics, then the specific checks and outputs. The price sentence is slightly long but earns its place because it prevents a blind first call. No filler sentences.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

With no output schema, the description compensates by naming the return values (grade, spoofable flag, fixes). Combined with the payment flow and latency note, an agent has everything needed to call this correctly on the first attempt.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Only one parameter ('domain'), and the input schema documents it fully, including that a URL or email address also works. The description adds no syntax or format details beyond the schema, so the baseline 3 is appropriate.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource ('Check a domain's email authentication and spoofing risk from public DNS') and enumerates the concrete checks (SPF, DMARC, DKIM selectors, MTA-STS, TLS-RPT, BIMI). Clear enough to distinguish it from siblings like security_headers or security_scan_lite.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

The use case is clear: assess a domain's email-spoofing posture from public DNS. It also gives practical invocation guidance for the x402 payment flow (first call returns terms, pay and retry with _meta). It does not explicitly say when to prefer this over overlapping siblings, but the context is well-defined.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.